1. Field of the Invention
The present invention relates to a circuit-containing photodetector that includes a photodetector element and a circuit for processing a photoelectrically converted signal supplied from the photodetector element both formed on the same semiconductor substrate. The invention further relates to a method of manufacturing the circuit-containing photodetector and to an optical device using the circuit-containing photodetector.
2. Description of the Background Art
Circuit-containing photodetectors are extensively applied to various optical devices such as optical sensors, optical pickups and photocouplers.
FIG. 21 schematically shows a cross section of a conventional circuit-containing photodetector. It is noted that in the drawings attached hereto, the same reference character denotes the same or corresponding components. Referring to FIG. 21, on a p-type silicon substrate 1 having a specific resistance of 100 xcexa9cm, for example, an n-type epitaxial silicon layer 2 of 5-8 xcexa9cm in specific resistance and 2-3 xcexcm in thickness is formed. A photodetector element 14 has a pn junction of p-type silicon substrate 1 and n-type epitaxial layer 2. On a light-receiving surface of photodetector element 14, an antireflection film 120 having a double layer structure of an oxide film 121 and a nitride film 122 is formed. In epitaxial layer 2, the region having photodetector element 14 and the region having a signal processing circuit portion 15 are electrically isolated from each other by a diffused isolation layer 13.
The signal processing circuit typically contains an npn transistor 15, which includes an n-type buried diffusion layer 8 buried in silicon substrate 1, n-type epitaxial layer (collector) 2 thereon, a p-type diffusion layer (base) 9 formed at the surface of the collector, an n-type diffusion layer (emitter) 6 formed at the surface of the base, and a collector compensation diffusion layer 11 extending from n-type buried diffusion layer 8 to the surface of epitaxial layer 2. On epitaxial layer 2, an interlayer insulating film 5, a metal electrode interconnect 4, a collector electrode 7 and an emitter electrode 10 are formed.
Recent years have seen increased density of optical recording, so that a shorter wavelength of signal light has been used for the optical recording. Accordingly, it is desired that the circuit-containing photodetector used for the optical recording should have a high sensitivity and a fast response to the short wavelength light. The conventional circuit-containing photodetector having the structure as shown in FIG. 21, however, can hardly achieve a higher sensitivity and a faster response for the reasons below.
If the signal light has a shorter wavelength, the absorption length of the light in the semiconductor becomes shorter. In silicon, for example, the absorption length is approximately 1 xcexcm for the light having a wavelength of 0.5 xcexcm, while it is 0.2 xcexcm or less for a wavelength of 0.4 xcexcm. In the semiconductor layer, the light absorbed into the depletion layer mainly contributes to the generation of photocurrent. The introduced light has to be absorbed in the vicinity of the pn junction to generate electron-hole pairs near the junction. Thus, when the signal light has a shorter wavelength, n-type epitaxial layer 2 is required to be thinner for efficient generation of the photocurrent. For example, if the light wavelength is 0.4 xcexcm, epitaxial layer 2 should have a thickness of 1 xcexcm or less. However, epitaxial layer 2 must be as thick as 2 to 3 xcexcm for forming npn transistor 15, so that photodetector element 14 has a low sensitivity to the employed short wavelength.
If a p-type impurity region having a junction depth of 1 xcexcm or less is formed at the surface of n-type epitaxial layer 2 in photodetector element 14, the photodetector element portion may have a high sensitivity to the short wavelength of the signal light. However, epitaxial layer 2 should have a low specific resistance (e.g. 5-8 xcexa9cm) for forming transistor 15, so that the photodetector element has a relatively high junction capacitance resulting in a slower response rate.
For the above reasons, the above conventional circuit-containing photodetector can hardly achieve both of a higher sensitivity and a faster response to a short wavelength light introduced into the photodetector element.
As a solution of such a problem, Japanese Patent Laying-Open No. 1-232774 discloses a method of producing a circuit-containing photodetector as mentioned below. In the method, as shown in FIG. 20A, an n-type epitaxial layer 2 having a specific resistance (5-8 xcexa9cm) and a thickness suitable for a transistor is grown on an n-type silicon substrate 1 having a specific resistance (80-100 xcexa9cm) and a thickness that are suitable for a photodetector element. At this time, an n-type high concentration buried layer 8 is also formed.
Referring to FIG. 20B, epitaxial layer 2 is partially etched away in the region that will provide a photodetector element, so that a trench is formed and silicon substrate 1 is partially exposed at the bottom of the trench.
Referring to FIG. 20C, an impurity region 3 is formed by diffusion at the bottom substrate surface of the trench, and simultaneously a base region 9 is formed by diffusion at the surface of epitaxial layer 2 in the region that will provide a transistor portion. Further, an emitter 6 is formed at the surface of base region 9 by diffusion.
In the circuit-containing photodetector as shown in FIG. 20C, photodetector element 14 can be formed to have both of a high sensitivity and a fast response to the short wavelength of signal light without the transistor characteristics degraded.
As mentioned above, epitaxial layer 2 should have a thickness of 2 to 3 xcexcm for forming therein a signal processing circuit such as a transistor. Accordingly, the depth of the trench should also be 2 to 3 xcexcm at photodetector element 14 formed in the circuit-containing photodetector as shown in FIG. 20C.
In this case, photodetector element 14 formed at the bottom of the trench needs a metal electrode interconnect 4 that extends from an anode 3 to the upper surface of epitaxial layer 2 rising 2 to 3 xcexcm out of the bottom of the trench.
Such a great rising in the trench, however, causes a problem as follows. When a pattern of metal electrode interconnect 4 is formed by means of photolithography, the light cannot uniformly be focused over the entire area of metal interconnect 4. The resulting line of metal interconnect 4 may have a narrowed width which is likely to cause a defect such as breaking of the line.
The present invention reflects the above-described problems in the conventional technique. Thus, an object of the present invention is to provide a structure of a circuit-containing photodetector that can not only have a high sensitivity and a fast response to signal light of short wavelength but be manufactured in high yield, and to provide a method of manufacturing such a circuit-containing photodetector.
According to the present invention, a circuit-containing photodetector is provided that has a semiconductor substrate, a photodetector element formed on the semiconductor substrate, and a circuit for processing an electric signal from the photodetector element. The circuit-containing photodetector includes (a) a semiconductor layer which is grown on the semiconductor substrate and on which the circuit is formed, (b) a trench which is formed in the semiconductor layer and has a depth to reach the semiconductor substrate, (c) an impurity region formed at a surface of the semiconductor substrate exposed at the bottom of the trench, the impurity region constituting the photodetector element, and (d) a conductive impurity region extending from the impurity region of the photodetector element to an upper surface of the semiconductor layer for transmitting the electric signal from the photodetector element to the circuit.
In the device according to the present invention, the conductive impurity region may be formed in the semiconductor layer to extend from the bottom of the trench to the upper surface of the semiconductor layer.
On the other hand, in the device according to the present invention, the conductive impurity region may be composed of a substrate impurity region formed at a portion of the semiconductor substrate that is adjacent to the trench, and a semiconductor layer impurity region formed in the semiconductor layer to extend from the substrate impurity region to the upper surface of the semiconductor layer. In this case, a buried diffusion layer for the circuit may be formed at a portion of the substrate distant from the substrate impurity region, or the substrate impurity region for transmitting a signal may also serve as a buried diffusion layer for the circuit. Further, a collector compensation diffusion layer for the circuit may be formed at a portion of the semiconductor layer distant from the semiconductor layer impurity region, or the semiconductor layer impurity region for transmitting a signal may also serve as a collector compensation diffusion layer for the circuit.
According to the present invention, the photodetector element may include a plurality of impurity regions of the photodetector element.
According to the present invention, is provided a method of manufacturing the circuit-containing photodetector as stated above. The manufacturing method includes the steps of (a) partially etching a semiconductor layer grown on a semiconductor substrate to form a trench in the semiconductor layer and (b) doping with impurities a portion of the semiconductor substrate exposed at the bottom of the trench and a side portion of the trench to form an impurity region that constitutes a photodetector element and to form a conductive impurity region for transmitting an electric signal from the photodetector element.
According to the present invention, is provided another method of manufacturing the circuit-containing photodetector as stated above. The manufacturing method includes the steps of (a) doping with impurities specific portions of a semiconductor substrate and a semiconductor layer grown on the semiconductor substrate and (b) partially etching the doped specific portions to form a trench. In the step of forming the trench, an impurity region that constitutes a photodetector element and a conductive impurity region for transmitting an electric signal from the photodetector element are formed in the remaining part of the specific portions.
According to the present invention, is provided still another method of manufacturing the circuit-containing photodetector as stated above. The manufacturing method includes the steps of (a) doping with impurities specific portions of a semiconductor substrate and a semiconductor layer grown on the semiconductor substrate, (b) partially etching the doped specific portions to form a trench, wherein a conductive impurity region for transmitting an electric signal from a photodetector element is formed in the remaining part of the specific portions, and (c) doping with impurities the bottom of the trench to form an impurity region that constitutes the photodetector element.
According to the present invention, is provided a further method of manufacturing the circuit-containing photodetector as stated above. The manufacturing method includes the steps of (a) doping with impurities a specific surface portion of a semiconductor substrate to form a first impurity region, (b) growing a semiconductor layer on the semiconductor substrate, (c) partially etching the semiconductor layer to form a trench in the semiconductor layer, (d) doping with impurities the bottom of the trench to form a second impurity region in contact with the first impurity region, and (e) doping with impurities a specific portion of the semiconductor layer to form a third impurity region extending from the first impurity region to an upper surface of the semiconductor layer. The second impurity region constitutes a photodetector element and the first and third impurity regions form a conductive impurity region for transmitting an electric signal from the photodetector element.
According to the present invention, is provided a still further method of manufacturing the circuit-containing photodetector as stated above. The manufacturing method includes the steps of (a) doping with impurities two adjacent surface portions of a semiconductor substrate to form a first impurity region and a second impurity region, (b) growing a semiconductor layer on the semiconductor substrate, (c) partially etching the semiconductor layer to form a trench in the semiconductor layer, and (d) doping with impurities a specific portion of the semiconductor layer to form a third impurity region extending from the first impurity region to an upper surface of the semiconductor layer. The second impurity region constitutes a photodetector element and the first and third impurity regions form a conductive impurity region for transmitting an electric signal from the photodetector element. In this manufacturing method, the first and second impurity regions may simultaneously be formed.
According to the present invention, another circuit-containing photodetector is provided that has a semiconductor substrate, a photodetector element formed on the semiconductor substrate, and a circuit for processing an electric signal from the photodetector element. The circuit-containing photodetector includes (a) a semiconductor layer which is grown on the semiconductor substrate and on which the circuit is formed, (b) a trench which is formed in the semiconductor layer and has a depth to reach the semiconductor substrate, (c) an impurity region formed at a surface of the semiconductor substrate exposed at the bottom of the trench, the impurity region constituting the photodetector element, and (d) a silicide film formed in self-aligning manner from the bottom of the trench to an upper surface of the semiconductor layer for transmitting the electric signal from the photodetector element to the circuit.
According to the present invention, is provided an optical device including the circuit-containing photodetector.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.